System and method for closed loop inspiratory pressure control in a ventilator

Abstract: The method and system for controlling airway pressure in a ventilator during the inspiratory cycle by controlling the flow of breathing gas through a flow supply valve at a desired rate of supply flow and venting the breathing gas at a desired rate of outflow through a relief valve to achieve the desired airway pressure. Measurements of the rates of actual supply flow, the actual outflow through the exhalation/relief valve, and the pressure in the airway are compared with desired values for determination the amount of error for feedback control of the ventilator system.

Description: BACKGROUND OF THE INVENTION Field of the Invention : This invention relates generally to breathing ventilators, and more particularly relates to a system and method for controlling patient airway pressure during the inspiratory cycle of a breath by control of a flow delivery valve and a relief valve in combination to achieve a desired pressure level. Description of Related Art : Medical ventilators are designed to assist a patient in breathing when the patient is somehow unable to adequately breathe on his own. In some form of ventilator systems, the patient is provided with ventilation support in the form of pressure assistance after the patient has began his inspiratory effort. With such a system, it is desirable to immediately increase the pressure after a breath is initiated in order to reach a target pressure input by the care provider. This rise in pressure causes flow to be initiated in the patient airway which supplies breathing gas to the patient's lungs. Conventional pressure controlled ventilator systems control the flow of gas to the patient by use of a pressure controller, a flow controller and flow valve in the inlet side of the airway, terminating the flow when the target pressure is reached. However, such a control strategy often results in over pressurization of the patient, with the pressure overshooting the target pressure by a greater or lesser amount depending on the physiology of the patient and the construction of the airway. Often, the overshoot is sustained, thus over pressurizing the patient's lungs for the entire inspiration cycle of the breath. When this occurs, the possibility exists that the patient will be harmed by the higher than desirable pressure in the lungs. For instance, if the patient has recently had thoracic or abdominal surgery, the possibility exists that such over pressures could rupture sutures or blood vessels recently repaired in the surgery. Similarly, a very frail or infirm patient such as one with asthma or emphysema could be harmed if the breath pressure is in excess of that desired. For these reasons, it would be desirable if the delivery of breaths in a pressure controlled ventilator could be controlled to eliminate overshoot or long term over pressurization in the patient airway. It would be particularly advantageous if the control of such overshoots could be accomplished without significant changes to the basic configuration and utilized existing components. One suggested solution to the problem of over pressuristation is described in US-A-4,527,557 to include the provision of an exhalation valve. However, as described, the exhalation valve operates simply as a safety valve, venting the system when an excess pressure is sensed. There is no suggestion of the exhalation valve being actively controlled. Nevertheless, US-A-4,527,557 does describe a medical ventilator system having the features set out in the pre-characterising portions of the accompanying claims. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a method of controlling airway pressure in a breathing gas circuit of a ventilator for a patient during the inspiratory cycle of a breath, the breathing gas circuit having a source of breathing gas, an airway flow path in fluid communication with a breathing attachment of the patient for supplying the breathing gas to the breathing attachment, a flow supply valve for controlling the supply of breathing gas to the breathing attachment, and a relief valve for controlling venting of breathing gas from the patient to the atmosphere, the method comprising the step of delivering a supply flow of breathing gas to the breathing attachment through the flow supply valve at a predetermined rate of supply flow based upon a desired airway pressure and actual airway pressure, characterised in that the method further comprises the steps of determining a desired rate of outflow, measuring the actual rate of outflow through the relief valve, comparing the actual rate of outflow with the desired rate of outflow to determine an outflow error, generating a relief valve command signal derived from a signal representing the desired airway pressure and a signal representing the outflow error to control operation of the relief valve, and, during the inspiratory cycle, venting the breathing gas from the breathing attachment through the relief valve in accordance with the relief valve command signal to achieve the desired airway pressure. By use of embodiments of the invention, improved control of the pressure in the patient airway is obtained, particularly with regard to controlling overshoot and sustained over-pressurisation during the inspiratory cycle. A current problem encountered during the inspiratory cycle of patient respirators is the control of overshoot of patient airway pressure during the inspiration cycle for pressure controlled type of ventilation. When the cycle is primarily controlled by the standard pressure and flow controllers, the patient is likely to experience an overshoot in the airway pressure as the controllers command the flow valve opening to rapidly supply breathing gas to the patient during the initial phases of the inspriation. Such an overpressure is often unavoidable if rapid filling of the patient airway is required and only control of the flow valve is available. Thus, it is highly desirable to provide more accurate control of patient airway pressure during the inspiratory cycle of the breathing cycle, since over-pressurisation of the patient's lungs has a number of undesirable effects. According to a second aspect of the present invention there is provided a system for closed loop control of airway pressure in a breathing gas circuit of a ventilator for a patient during the inspiratory cycle of a breath, the breathing gas circuit having a source of breathing gas, and an airway flow path in fluid communication with a breathing attachment of the patient for supplying the breathing gas to the breathing attachment, the system comprising flow supply valve means in said airway flow path for delivering a supply flow of breathing gas from the source of breathing gas to the breathing attachment at a desired rate of supply flow based upon a desired airway pressure and the actual airway pressure; and relief valve means for the venting of breathing gas from the breathing attachment to the atmosphere during the inspiratory cycle, characterised in that the system further includes means for determining a desired rate of outflow, means for measuring the actual rate of outflow through the relief valve means, means for comparing the actual rate of outflow with the desired rate of outflow to determine an outflow error, and means for generating a relief valve command signal derived from a signal representing the desired airway pressure and a signal representing the outflow error to control operation of the relief valve means to vent the breathing gas from the breathing attachment through the relief valve means during the inspiratory cycle to achieve the desired airway pressure. Using the system described, the pressure in the patient airway can be controlled to avoid overshoot, even if very aggressive initial flow rates are used to insure rapid filling of the lungs during the first stages of the inspiration cycle. According to a third aspect of the present invention there is provided an apparatus for the control of airway pressure in a ventilation respirator of a patient according to claim 7. In a presently preferred embodiment of this aspect of the invention, the desired patient airway pressure is summed with the measured patient airway pressure to create an error signal to be used as an input to a pressure controller. The pressure controller outputs a flow control signal representing the desired flow to achieve the desired pressure. The flow control signal is summed with the output of the flow valve to provide an error signal used to drive the flow valve controller. A relief valve, which is preferably an adjustable, actively controlled valve, is connected to the patient airway and driven by a command signal derived from a signal representing a desired pressure and a relief flow error signal. The relief error signal is based on the difference between the measured relief valve flow and the desired relief valve flow, which may be established based on a number of factors related to system and medical parameters. For example, patients are often medicated by nebulizers and the like, which results in the presence of unabsorbed medicants in the exhaled breathing gas. In such a case it may be desirable to limit the relief valve flow to a relatively low level such as 1 Liter/minute in order to limit exposure of others to the patient's medication. In general, it is desirable to limit the relief valve flow to limit the usage of gasses by the respirator. In a presently preferred embodiment, the relief valve may advantageously be an exhalation valve of the force balance variety well known in the art. Such valves may be controlled by electrical, pneumatic or mechanical inputs. From the above, it can be seen that embodiments of the present invention provide an improved method and apparatus for the control of patient airway pressure during the inspiratory cycle of the patient breath on a ventilator by actively controlling both the flow control valve and a relief valve, which can be an exhalation valve, thereby providing both a rapid rise time of pressure and flow and a reduction in overshoot of airway pressure past a desired target pressure. Other properties and advantages of the invention will be apparent from the following detailed description and the accompanying drawings, which illustrate, by way of example, the features of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a prior art ventilator pressure controlled inspiratory breath control system; Fig. 2 is schematic diagram of an embodiment of the pressure controlled ventilator inspiratory breath control system of the invention; Fig. 3a is an illustration of a balloon type force balance valve; Fig. 3b is an illustration of a diaphragm-type force balance valve; Fig. 4(a) is an illustration of patient pressure during a pressure controlled inspiration phase using a prior art controller; Fig. 4(b) is an illustration of patient flow during a pressure controlled inspiration phase using a prior art controller; Fig. 5(a) is an illustration of patient pressure during a pressure controlled inspiration phase using the control system of Figure 2; and Fig. 5(b) is an illustration of patient flow during a pressure controlled inspiration phase using the control system of Figure 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Pressure controlled ventilation systems provide pressure supported flow of breathing gas to a patient after the ventilator has sensed the initiation of a breath by the patient. However, if the flow of gas is not precisely controlled, the patient airway, and thus the patient's lungs, can be over pressurized by a pressure rise above the target level. In such an event, the patient can experience discomfort or possible harm from the effect of the higher pressure on the respiratory system. A prior art pressure controlled ventilator control system illustrated in Fig. 1 includes a means to input a desired pressure to an error summing means 2 which also receives an input from a pressure sensor 4 measuring patient pressure in the patient airway system 6. The difference between measured pressure and desired pressure is an error signal which is used as an input to the pressure controller 8 which generates a signal representing desired flow rate. The desired flow rate is summed with the signal from flow sensor 16 by summing means 10 to create a flow error signal used as an input to flow valve controller 12, which generates a flow valve command to drive flow valve 14. Flow valve 14 controls flow to the patient airway and tubing system 6. Using such a system, the reservoir represented by the patient airway and the patient's lungs is filled rapidly after initiation of a breath with pressure support. Ideally, gas flow should be cut off as soon as a target pressure is reached; however, the dynamic response of prior art controllers and gas delivery systems often results in an overshoot of pressure resulting in over pressurization of the patient. The present invention is embodied in a system and method for actively controlling both the inspiratory flow and pressure and a relief valve in the patient circuit system to prevent significant sustained overshoot of pressure beyond the target set by the care provider. As illustrated in Fig. 2, one embodiment of the present invention utilizes a pressure controlled ventilation system of the type illustrated in Fig. 1, along with active control of a relief valve, which could be the exhalation valve, to control the airway pressure. More specifically, summer 2 outputs an error signal which is used as an input to the pressure controller 8 on the basis of an input desired pressure and the measured airway pressure from pressure sensor 4. Pressure controller 8 outputs a desired flow rate which is summed by summing means 10 with the flow rate measured by flow sensor 16. The output of summing means 10 is used to drive flow valve controller 12 and thus flow valve 14 to provide a rate of flow to the patient through patient airway 6. The embodiment also inputs the desired pressure to an exhalation valve controller 20, the output of which is summed by summing means 22 to create an exhalation valve command to drive the exhalation valve 18. The other input to summing means 22 is the output of exhalation valve flow controller 24, which is in turn driven by the output of summing means 26. Summing means 26 outputs a relief flow error signal input to exhalation valve flow controller derived from a desired relief flow and the exhalation valve relief flow measured by exhalation valve relief flow sensor 28. By use of the embodiment, overshoot of pressure in the patient airway is actively controlled by the exhalation valve on the basis of sensed flow and desired pressure. Another benefit of the embodiment is that a desired pressure trajectory may also be input and the trajectory will be accurately tracked using the closed loop system. Thus, a variety of respiration strategies other than mere fixed level target pressure may be accommodated by the embodiment. While a variety of relief valves may be used with the invention, one particular form of force balanced valve utilized as exhalation valves has been shown to be advantageous. Figs. 3a and 3b illustrates two types of such exhalation valves, which are of the balloon or diaphragm type. The balloon type valve illustrated in Fig. 3(a) incorporates a balloon 30 inflated by pressure from pilot pressure inlet 32, the expiratory flow will open the valve when the expiratory flow force on the valve exceeds the force created from the pilot pressure. Similarly, the diaphragm valve of Fig. 3(b) is provided with pilot pressure inlet 32 operating on the cavity formed by the valve exterior body, the valve poppet and the area behind diaphragm 34, thus controlling expiratory flow similarly to balloon valve 3(a). While it has been shown that such valves are useful for the purposes of the relief valve, other valves which are electrically, mechanically or pneumatically driven, or a combination of these drive schemes, can be advantageously utilized for the relief valve. From the foregoing, it should be clear that embodiments of the invention provide for an improved pneumatically driven, electronically controlled, pressure control system for a ventilator system for providing breathing gas to a patient at a desired flow rate and pressure level. More particularly, these embodiments allow prevention of overshoot of patient airway pressure over the target pressure set by a care provider by allowing for the incorporation of target pressure flow trajectories of more sophisticated ventilation strategies. The use of the invention in combination with prior art ventilators has demonstrated substantial improvements in performance. Figs. 4(a) and 4(b) illustrate the pressure and flow time histories for a ventilator inspiration phase utilizing prior art controllers. More specifically, Fig. 4(a) illustrates the patient pressure in cm of H 2 O at the patient wye for a commanded and desired pressure of 30 cm H 2 O. As will be noted from this trace of pressure produced by prior art controllers, the patient airway pressure persists at a pressure on the order of 7 cm of H 2 O above the desired pressure for a substantial portion of the breath after the beginning of inspiration. While this overshoot can be somewhat moderated if a slower initial inspiration rate is used, such a strategy results in less rapid filling of the lungs during the inspiration, and thus inferior ventilation. As illustrated in Fig. 4(b), when compared with Fig. 4(a), the patient overpressure persists long after the delivery of a breath is completed, an undesirable result of the use of prior art controllers. Figs. 5(a) and 5(b) illustrate the improved performance of a system utilizing the control system of the present invention. Referring to Fig. 5(a), a rapid rise time is utilized, as shown in Fig. 4(a), to facilitate rapid filling of the lungs of the patient once a breath is begun. Unlike the prior art controllers, however, the present invention is able to reduce the sustained overshoot of pressure after lung filling, and airway pressure can be maintained at the desired pressure of 30 cm H 2 O. Referring to Fig. 5(b), the time history of flow rate flow to the patient is illustrated, as is the flow from the relief valve. It can be seen that the relatively modest flow rates from the relief valve near the early peaking of pressure can allow rapid return of patient airway pressure to the desired level. Additionally, as shown by the graph at the 1.2 second mark, the relief valve can be used to rapidly reduce pressure in the airway for the beginning of the expiration cycle. Thus, the use of the invention substantially reduces the sustained overpressure of the patent when compared to prior art controllers. It should be recognized that although the invention has been described as providing individual, separate pressure, flow, and exhalation controllers, it is also possible that these functions can be performed by one or more microprocessors with appropriate inputs from the flow and pressure sensors provided, and outputs to the supply flow valve and pressure regulator mechanism. It will be apparent from the foregoing that while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.

Claims: A method of controlling airway pressure in a breathing gas circuit (6) of a ventilator for a patient during the inspiratory cycle of a breath, the breathing gas circuit (6) having a source of breathing gas, an airway flow path in fluid communication with a breathing attachment of the patient for supplying the breathing gas to the breathing attachment, a flow supply valve (14) for controlling the supply of breathing gas to the breathing attachment, and a relief valve (18) for controlling venting of breathing gas from the patient to the atmosphere, the method comprising the step of delivering a supply flow of breathing gas to the breathing attachment through the flow supply valve (14) at a predetermined rate of supply flow based upon a desired airway pressure and actual airway pressure, characterised in that the method further comprises the steps of determining a desired rate of outflow, measuring the actual rate of outflow through the relief valve (18), comparing the actual rate of outflow with the desired rate of outflow to determine an outflow error, generating a relief valve command signal derived from a signal representing the desired airway pressure and a signal representing the outflow error to control operation of the relief valve (18), and, during the inspiratory cycle, venting the breathing gas from the breathing attachment through the relief valve (18) in accordance with the relief valve command signal to achieve the desired airway pressure. The method of Claim 1, further including the steps of measuring the actual rate of supply flow from the flow supply valve (14) to the breathing attachment, comparing the actual rate of supply flow with the predetermined rate of supply flow to determine a supply flow error, and generating a supply valve command signal based upon the supply flow error to control operation of the flow supply valve (14) to deliver the breathing gas at the predetermined rate of supply flow. The method of Claim 1, further including the steps of measuring the actual airway pressure, comparing the actual airway pressure with an instantaneous desired airway pressure to determine an airway pressure error, and modifying the desired rate of flow based upon the airway pressure error. A system for closed loop control of airway pressure in a breathing gas circuit (6) of a ventilator for a patient during the inspiratory cycle of a breath, the breathing gas circuit (6) having a source of breathing gas, and an airway flow path in fluid communication with a breathing attachment of the patient for supplying the breathing gas to the breathing attachment, the system comprising flow supply valve means (14) in said airway flow path for delivering a supply flow of breathing gas from the source of breathing gas to the breathing attachment at a desired rate of supply flow based upon a desired airway pressure and the actual airway pressure; and relief valve means (18) for the venting of breathing gas from the breathing attachment to the atmosphere during the inspiratory cycle, characterised in that the system further includes means for determining a desired rate of outflow, means (28) for measuring the actual rate of outflow through the relief valve means (18), means (26) for comparing the actual rate of outflow with the desired rate of outflow to determine an outflow error, and means (20, 22, 24) for generating a relief valve command signal derived from a signal representing the desired airway pressure and a signal representing the outflow error to control operation of the relief valve means (18) to vent the breathing gas from the breathing attachment through the relief valve means (18) during the inspiratory cycle to achieve the desired airway pressure. The system of Claim 4, further including means (16) for measuring the actual rate of supply flow from the flow supply valve means (14) to the breathing attachment, means (10) for comparing the actual rate of supply flow with the desired rate of supply flow to determine a supply flow error, and means (12) for generating a supply valve command signal based upon the supply flow error to control operation of the flow supply valve means (14) to deliver the breathing gas at the desired rate of supply flow. The system of Claim 4, further including means (4) for measuring the actual airway pressure, means (2) for comparing the actual airway pressure with the desired airway pressure to determine an airway pressure error, and means (8) for modifying the desired rate of flow based upon the airway pressure error. An apparatus for the control of airway pressure in a ventilation respirator of a patient, the apparatus comprising means to set a desired target airway pressure; a pressure controller (8); a flow valve controller (12); a breathing gas flow valve (14); a sensor (16) for measuring the output flow of breathing gas from said breathing gas flow valve (14); a patient tubing airway system (6) in fluid communication with said flow valve (14) and the patient respiratory system; a pressure sensor (4) in fluid communication with said patient tubing airway system (6); first summing means (2) to create an error signal between the predetermined airway pressure and the signal from the pressure sensor (4), said error signal providing the signal to drive the pressure controller (8); second summing means (10) to determine the error between the output signal from the pressure controller (8) and the output signal from the patient airway flow sensor (16), said error signal providing the input to said flow valve controller (12); and a relief valve (18) in fluid communication with said patient tubing airway system (6), characterised in that the apparatus further comprises a relief valve flow sensor (28) in fluid communication with the output flow of said relief valve (18); a relief valve pressure controller (20) which outputs a command signal based upon a desired airway pressure; a third summing means (26) which outputs an error signal based upon the difference between a desired relief flow and the relief valve flow measured by the relief valve flow sensor (28); a relief valve flow controller (24) which outputs a relief valve command signal based upon the error signal from said third summing means (26); and fourth summing means (22) to output a relief valve command signal to said relief valve (18) to vent flow during the inspiratory cycle of a breath based upon the sum of the output signals of said relief valve pressure controller (20) and said relief valve flow controller (24) for controlling said relief valve (18) so as to achieve said desired target airway pressure.

Inventors: ISAZA FERNANDO J, WONG STANLEY Y, ISAZA, FERNANDO J., WONG, STANLEY Y.
Applicants: PURITAN BENNETT CORP, PURITAN-BENNETT CORPORATION
Application year: 1994
Main classification: A61M16
IPC codes: A61M16